Films for energy-saving sheet materials in greenhouses must comply with a large number of requirements. Firstly, that portion of the light that is required for plant growth should pass through the film/energy-saving sheet material, and during the night and in particular in the morning hours the energy-saving sheet material should moreover retain the heat that rises from the soil, not only by retarding convection but also by reflection of radiation within the greenhouse. In the absence of the energy-saving sheet material, energy consumption within the greenhouse increases, and it becomes more difficult to establish the ideal conditions of temperature and humidity. However, these materials generally have the disadvantage that solar radiation is interrupted by an additional layer which, both by absorption and by reflection, reduces the amount of light available. In the period around midday, the energy-saving sheet material can be raised, or indeed it can become necessary to use energy-saving sheet materials for cooling because of excessive incident light. However, the energy-saving sheet material is particularly important in the morning hours because the temperature required for plant growth must be achieved during this period and at the same time the amount of light made available must be maximized in order to ensure a high level of photosynthesis. However, in particular in the morning hours the sun is still at a small angle to the horizon, and this leads to even greater reflection at a film surface than when the sun is at a higher position. Reflection must therefore be reduced in particular at the main usage time of the materials.
The UV resistance of the film must moreover allow the energy-saving sheet material to remain in use in a greenhouse for at least 5 years while not exhibiting any significant yellowing or embrittlement or cracking on the surface, or undergoing any serious deterioration of mechanical properties, or suffering any significant loss of transparency.